The present invention relates generally to an imaging system utilizing imaging sheets having a surface coating of rupturable photosensitive microcapsules, and more particularly, to a method for processing such imaging sheets to obtain an improved pressure roller developed image.
U.S. Pat. Nos. 4,399,209 and 4,440,846, which are assigned to The Mead Corporation and are herein incorporated by reference, describe an imaging system wherein a photosensitive layer comprising microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase. An image-forming material, such as a substantially colorless color former, is typically associated with the microcapsules. When the microcapsules rupture, the color former image-wise reacts with a developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing the image-wise exposed imaging sheets through the nip between a pair of pressure rollers.
One of the problems encountered in developing the afore-said imaging sheets is that of achieving uniform developing characteristic. After the imaging sheet has been image-wise exposed, contact between the rollers and the imaging sheet surface is believed to be diminished where the microcapsules have been hardened by exposure, and thus, the rollers bridge these exposed microcapsule areas and some of the image detail is lost. The imaging sheet develops unevenly and the tonal characteristics of the resulting images are not entirely satisfactory.
When high pressures are applied to the calendar rollers as required to overcome roller surface irregularities and achieve uniform development across the roller, some of the midtone quality of the developed images is lost. Such a loss due to a loss of differential microcapsule rupture is referred to as midtone mottle. Thus, current processes for developing images through pressure rollers have resulted in images with poor tonal characteristics, midtone mottle and unacceptable highlight detail.